In a P2MP communication system, multiple optical network units are connected to a single optical line terminal through optical fibers. The optical network units are not always performing communication and have halt time. Hence, for the purpose of reducing the power consumption of an optical network unit, there is a suggestion that the optical network unit is placed in a halt state (see, for example, Patent Literature 1). In this case, for example, the optical network unit needs to wake up from the halt state after a certain period of time and perform normal operation. However, unless a source clock is stably generated before a clock and data recovery unit of the optical network unit returns to a normal operating state from a halt state, communication may fail. Hence, to prevent such events, those devices involved in phase-lock control for clock and data recovery are placed on standby in a normal operable state until a source clock is stably generated.